Ecology of North American Freshwater Fishes. Stephen T. Ross Ph. D.
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Assessing Assemblage Change
The stability and persistence of assemblages should be investigated on multiple levels within the hierarchical framework (Rahel 1990). Separating actual changes in species presence or absence from artifacts of sampling also is a pervasive and significant problem (Magnuson et al. 1994). Preferably, the goal of current research should not be to determine whether local assemblages change or not—virtually all local assemblages undergo change as individuals are added or removed (due to natality and mortality, or movements). Instead, as the temporal extent of data increases (several to many samples over a period of years to decades), and as change is measured on multiple spatial scales, it becomes of more interest to ask how much a local fish assemblage, or distribution of fish species within a watershed, have changed during various intervals (Ross and Matthews, in press). Also, an important issue is whether changes largely are driven by major events such as extensive droughts or floods, with little change during times that lack apparent disturbance factors. In other words, are the changes that can be observed in fish assemblages over long periods of time related more to gradual changes, or to dramatic events that may (or may not) leave their mark for years or more?
Dealing with Environmental Change
Fish populations can deal with environmental or biotic stressors in three primary ways. First, populations might lack means of dealing with perturbations and be eliminated from a region altogether. Second, individuals in a population might show resistance by withstanding such challenges through morphological, physiological, or behavioral adaptations, such as refuge-seeking behavior that would overall increase their tolerance to environmental perturbations. Finally, populations might emigrate from, or perish in, the stressed habitat but recover following a perturbation by return immigration of the displaced individuals or colonization by individuals from other populations, such that pre- and postdisturbance assemblage structures are the same or similar. This approach to dealing with environmental change was termed adjustment stability by Connell and Sousa (1983) and resilience by Dodds et al. (2004). Connell and Sousa (1983) considered that this response included two components: amplitude and elasticity. Amplitude is a measure of how far a population or assemblage can be displaced from its predisturbance state and still return; elasticity, drawing further on the analogy with a rubber band, is a measure of how quickly populations or assemblages can return to a predisturbance condition.
Resistance
It is not surprising that fish assemblages occurring in geographic regions prone to extreme climatic conditions are generally persistent in the face of such environmental challenges, given that these faunas have been under strong, long-term selection to deal with such conditions, and intolerant species would have been extirpated (C. L. Smith and Powell 1971). Examples of fish faunas inhabiting areas prone to disturbance include faunas of Great Plains streams. Fishes in these streams are subjected to periods of low flow and even dewatering as well as to intense periods of flooding. Such conditions likely occurred before widespread human-caused changes in the nineteenth and twentieth centuries, although the amount of silt has probably increased as a consequence of changes in land use (Matthews 1988).
In response to this environment, some fishes have evolved increased tolerances to low dissolved oxygen levels and high temperatures. Comparisons of physiological tolerances among minnow species from more benign upland streams in Arkansas with those inhabiting apparently harsher Great Plains streams in central and western Oklahoma generally showed that minnows from harsh environments were more resistant (Matthews 1987). As a group, Matthews showed that the minnows from the harsh streams had significantly greater tolerance to high water temperatures compared to fishes from the relatively benign streams, although one prairie minnow, the Emerald Shiner (Notropis atherinoides), had a critical thermal maximum (CTM) more in line with the upland fishes (Figure 6.1). However, Emerald Shiners, along with three other plains minnows, showed better survival at low oxygen levels compared to upland fishes or Blacktail Shiner (Figure 6.1).
FIGURE 6.1. Contrasts in physiological resistance of minnows from relatively harsh versus benign environments. Harsh environments include prairie streams from central and western Oklahoma; benign environments include streams from upland regions of Arkansas. Oxygen tolerance was measured as the percentage of fish surviving 8.5–10 h at low oxygen levels (0.2=0.9 ppm dissolved oxygen). Based on data from Matthews (1987).
Pupfishes (genus Cyprinodon) occur widely in fresh and brackish water habitats in Mexico (Miller 2005), in coastal brackish water areas along the Gulf of Mexico and Atlantic coasts (Johnson 1980; Nordlie 2003), and in desert regions of the southwestern United States (Naiman and Soltz 1981). In all regions, Cyprinodon species show high resistance to temperature extremes (Feldmeth 1981; Bennett and Beitinger 1997; Nordlie 2003). For instance, the Sheepshead Minnow (Cyprinodon variegatus) of the Atlantic and Gulf coasts can survive temperatures from a low of −1.8° C to a high of 43° C, the widest temperature range of any of the over 200 estuarine/salt marsh fishes reported by Nordlie (2003). Populations of cyprinodont fishes inhabiting the Death Valley region of Nevada and California also have wide temperature tolerances, being able to withstand temperatures of < 1° C to 40–44° C (Brown and Feldmeth 1971; Soltz and Naiman 1978; Feldmeth 1981).
Refuge-seeking behavior also allows fishes to resist adverse conditions in their environment and can have a role in surviving floods and droughts. Fishes in a variety of regions increase their resistance to downstream displacement during floods, especially during the winter when lowered water temperature limits their swimming ability, by actively selecting habitats with large structure such as woody debris, rocks, or other large and relatively immovable structures. Cutthroat Trout (Oncorhynchus clarkii) in a tributary of the Smith River in California showed twice the site fidelity in pools with large woody debris compared to those without (Harvey et al. 1999). During a winter flood event, trout in pools with large woody debris tended to remain in those pools in contrast to the greater movement shown by trout in less complex habitats. Fishes in an arid eastern Oregon stream also were more resistant to floods in structurally complex habitats compared to simple habitats (Pearsons et al. 1992). In the southeastern United States, Bayou Darters (Nothonotus rubrum), a species endemic to the Bayou Pierre system of Mississippi, responded to winter water temperatures (7.5–11° C) in an artificial stream by shifting their distribution to habitat patches with larger particle sizes of coarse gravel and pebbles (Figure 6.2A). In addition, under winter conditions, as flow increased from 14 to 35 cms-1, Bayou Darters selected habitat patches with large refuges to current (in this case bricks 14 × 7 × 7.5 cm) over habitats with just pebbles (Figure 6.2B).
FIGURE 6.2. Behavioral resistance of Bayou Darters (Nothonotus rubrum) to high winter stream flow in western Mississippi.
A. At cold temperatures (7.5=11.0° C) in a laboratory stream with a current speed averaging 31 cms-1, Bayou Darters shift their habitat selection to patches with larger particle sizes compared to habitat selection at warm temperatures (22° C).
B. Even in habitats with large substrata, Bayou Darters select patches with larger refuges (bricks) at moderate versus low current speeds. In both figures, bars are 95% confidence intervals. Based on data from Ross et al. (1992b).
FIGURE 6.3. Behavioral resistance of adult and one-day-old Sonoran Topminnow (Poeciliopsis occidentalis) to displacement by floods, compared to the nonnative Western Mosquitofish (Gambusia